Clojure: Do something for list and string

Question

Context: let's create a percent encoder. So, characters will be encoded to theirs % representations. Also, I would like vector of string will be encoded like this: [a b] -> (str (encode a) "+" (encode b)). The reason behind is I want spaces be encoded to %20 for strings and lists of strings cat-ed with +. I made this using protocols:

(defprotocol IUrlEncodable
  (^String url-encode [v]))

(extend-protocol IUrlEncodable
  String
  ;; Do I need type here? I have it in the protocol.
  (^String url-encode [v]
    (-> v URLEncoder/encode (.replace "+" "%20")))
  IPersistentVector
  ;; Same question here.
  (^String url-encode [v]
    (->> v (map url-encode) (join "+"))))

Is it a canonical way to do it? Did I made right thing? What are alternatives? Do I need to redeclare type hint in protocol extension, if I have declared it in the protocol?

Michał Marczyk · Accepted Answer · 2013-09-22 00:34:15Z

3

Only the type hint on the protocol function matters; the type hints in method implementations have no purpose. Strictly speaking, the compiler will use type hints attached to Vars holding functions. The var in question here -- #'url-encode -- is created by the defprotocol form (along with #'IUrlEncodable, which holds some protocol administrativia, and a related JVM interface). Metadata attached to the url-encode method name will be transferred to this Var. Metadata attached to symbols naming this method in calls to extend & Co. has no relevance.

Note that technically you don't need any type hints at all, though if you do put them in, they may help the compiler to avoid reflection in some scenarios.

answered Sep 22, 2013 at 0:34

Michał Marczyk

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4 Comments

Michał Marczyk Over a year ago

No, the type hint attached to url-encode as in the function text gets attached to the #'url-encode Var and is interpreted as pertaining to the return values. Type hints attached to arguments are a different matter -- these actually need to be specified in the implementations. (NB. no type hint should be specified on the leading "this" argument, since its type is known.)

Michał Marczyk Over a year ago

(The above was in answer to a comment which was apparently deleted. The question was whether the type hint applies to the parameter v as opposed to the function url-encode.)

demi Over a year ago

Overall, is my code idiomatic? I mean is protocol suitable here?

Michał Marczyk Over a year ago

Yes, using a protocol is fine. Though if you only care about strings and vectors of strings, a simple function with a vector? check would do just as well.

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